1. Field of the Invention
This invention relates to a microwave choke and in particular to a hybrid choke including lumped and distributed elements.
2. Description of Related Art
A choke is often used when it is desired to isolate microwave-frequency energy from low-frequency energy. A typical example is the isolation of a power supply from microwave energy propagated in circuitry powered by the supply.
In some uses, such as in portable cellular telephones, it is important that a microwave choke be as compact as possible. A small-sized microwave choke can be made from lumped capacitive and inductive elements, but the inductive-element Q is generally too low for efficient operation. In other words, lumped elements with suitably high inductances for use in microwave chokes also generally have resistances which dissipate more power than is acceptable. This is especially a problem in portable microwave apparatus where such dissipation both reduces battery life and decreases the power output of the apparatus. In the example given, i.e. portable cellular telephones, the consequence of such unacceptable power dissipation is reduced talk time and transmission range.
Alternatively, a distributed inductive element, such as a quarter-wave transmission line, can be used in place of a lumped inductive element in a microwave choke. Especially at the low end of the microwave frequency band, however, a quarter-wave transmission line can be relatively large and occupy more space than is available. This is particularly true in certain portable apparatus, such as in portable cellular telephone apparatus operating in the frequency range of 800 to 2400 MHz.